Process and assembly for non-destructive surface inspections

ABSTRACT

A light beam is directed towards a surface along a direction normal to the surface. The surface is caused to move so that the beam scans the surface along a spiral path. An ellipsoidal mirror is placed with its axis along the surface normal to collect light scattered by the surface and any anomalies at the surface at collection angles away from the surface normal. In some applications, a lens arrangement with its axis along the surface normal is also used to collect the light scattered by the surface and any anomalies. The light scattered by the mirror and lenses may be directed to the same or different detectors. Preferably light scattered by the surface within a first range of collection angles from the axis is detected by a first detector and light scattered by the surface within a second range of collection angles from the axis is detected by a second detector. The two ranges of collection angles are different, with one detector optimized to detect scattering from large particles and defects and the other detector optimized to detect light from small particles and defects.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/619,109,filed Jul. 10, 2003; which application is a continuation of applicationSer. No. 09/901,998, filed Jul. 10, 2001, now U.S. Pat. No. 6,606,153;which application is a continuation of application Ser. No. 08/770,491,filed Dec. 20, 1996, now U.S. Pat. No. 6,271,916; which application is acontinuation-in-part of application Ser. No. 08/533,632, filed Sep. 25,1995, now abandoned; and also a continuation-in-part of application Ser.No. 08/216,834, filed Mar. 24, 2004, now abandoned. These applicationsare incorporated by reference in their entirety as if fully set forthherein.

BACKGROUND OF THE INVENTION

The invention described in the present disclosure relates to a processand an assembly for the non-destructive inspection of surfaces,particularly for the measurement of small particles, defects, andinhomogeneities on and/or just below the surface of a test object, suchparticles, defects, and inhomogeneities collectively referred to hereinas anomalies. It relates in particular to an instrument described belowas the preferred embodiment for inspecting a silicon wafer, theinstrument having a light source that generates a light beam, a beamdeflector, an optical system that projects the incident beam on a lightspot perpendicular to the test object, a photodetector to which thecollected light is guided, and an assembly by which the test object ismoved by a coordinated translational and rotary movement, so that thelight spot scans the whole of the surface along a spiral path.

Such types of process and/or assembly can be used, for example inmicroelectronics, for the non-destructive checking and inspection of thesurfaces of wafers, magnetic storage media, and/or substrates foroptical applications, to determine the presence of any particles and/ordefects.

The development of wafer-exposure processes has made it possible tomanufacture wafer surfaces with ever finer structures parallel to thisdevelopment, inspection systems that permit the detection of ever moreminute defects and particles have become increasingly important. Apartfrom particles that account for about 75% of all waste in themanufacture of integrated circuits (ICS), inspection systems must becapable of detecting many other types of inhomogeneity, such asvariations in the thickness of coatings, crystal defects on and belowthe surface, etc.

In the final inspection by wafer manufacturers and the inward-goodsinspection by chip manufacturers, the unstructured, uncoated wafer musttherefore be subjected to extremely searching examination for particlecontamination, light-point crystal defects, roughness, polishingscratches, etc. If the test object has a rough surface, then a largeamount of stray surface scattering will result. Thus, for this purpose,the test object has a well-polished surface that produces very littlediffused light.

In chip manufacture it is usual to monitor each stage of the process, inorder to recognize problems as early as possible and thus avoid unduewaste. One method of process monitoring is to use so-called monitorwafers which remain unstructured but pass through some of the processstages. Comparison of two measurements, the first before the processstage and the other after it, can thus, for example, help determine theamount of particle contamination due to that process stage or indicatevariations in the evenness of the process stage, for example thedistribution of the coating thickness over the whole of the wafer. Thesurfaces subjected to inspection may be rough and metallized, andtherefore, produce a great deal of diffused light, or they may befilm-coated surfaces that cause interference-fringe effects. Thus,ideally the inspecting instrument has a wide dynamic range to permitdefect and particle detection of a wide variety of surfaces.

PRIOR ART

For the type of inspection described above, so-called laser scanners areparticularly suitable. An important feature of these is their highsensitivity to very small defects and the ability to determine thepresence of these, and their high throughput. The main differences inthe laser scanners now available are the type of scanning they use,their optical configuration, and the manner in which the results areprocessed.

For applications that require a high throughput and 100% inspection ofthe whole wafer surface, two processes are mainly used. In the first ofthese, for example as described in U.S. Pat. No. 4,314,763, theilluminating beam and the collecting optics are stationary, and the testobject is scanned spirally by means of a coordinated translational androtary movement of the test object itself. In the second process, forexample as described in U.S. Pat. No. 4,378,159, a rotating or vibratingmirror moves the illuminating beam in one direction linearly back andforth across the wafer, and the whole of the wafer is scanned by virtueof a simultaneous translational movement of the test objectperpendicular thereto.

Spiral scanning has the following advantages:

-   -   the optical system has no moving parts and thus is simpler;    -   the illuminated spot and the collector system's field remain        constant during the whole of the measurement procedure, hence        the system's sensitivity is homogeneous over the whole of the        test object;    -   the system takes up less room, because the test object has to be        moved only by the length of its radius; and    -   there is no need to alter the optical system for inspection of        bigger objects, only the travel of the translational stage.

The advantages of moving the illuminating beam by means of a mirror or aset of mirrors are:

-   -   the test object has to be moved in one direction only, and this        is simpler; and    -   as a rule, scanning is faster.

In the second scanning method, because the illuminating spot movesacross the test object and thus the source of diffused light moves inrelation to the optical collector system, it cannot ensure an evenmeasuring sensitivity, nor does it permit a rotationally symmetricalarrangement of the collector optics. These are serious drawbacks inlaser scanners configured in this manner.

Various optical configurations are known from prior art in the use of alaser scanner for spiral scanning as described above.

For example, U.S. Pat. No. 4,893,932 describes a system which has twodifferently polarized lasers and two corresponding detectors. Thediffused-light intensity of spheres as a function of their diameter hasoscillations for diameters within the range of the wavelength used andincreases strictly monotonically for smaller diameters. The use ofdifferently polarized light reduces the error in the attribution ofdiffused-light intensity to particle diameter for the spheres ofpolystyrene latex (PSL) spheres used for the calibration of laserscanners.

But in practice, the attribution of certain diffused-light intensitiesto particle diameters depends on so many factors, such as substratematerial, films and coatings available, particle material, surfacetexture of particles, etc., that when the optics and calibration of theequipment are designed only for polystyrene-latex spheres, they tend tomake interpretation of the results more difficult. A further majordrawback of this method is that the oblique angle of incidence andlinear polarization of the laser beam break the symmetry. The measuredsignal thus depends on the orientation of the defect.

Japanese Patent Application No. 6-314830 describes collector optics madeup of concentric rings, each having six fibre-optic light guides, whichare directed to a photomultiplier. The drawbacks of this arrangement arethat it fails to cover the central zone near the axis, and the discretearrangement can achieve rotational symmetry only approximately.

EP-A-0290228 describes an arrangement whereby the diffused light isconducted to two detectors. The first detector collects light deflectedby about 40 mrad to 100 mrad, the second collects light diffused by morethan 100 mrad. Such an angle-resolving method of measurement by means oftwo detectors makes it possible to classify the defects, but because thecollector angle is limited, the system cannot measure very smalldefects.

DE-A4,134,747 describes a similar solution that uses two detectorsdesigned as arrays, one of which measures the radial and the other theazimuthal light distribution. In this system the test object rotates andthe optical system moves linearly.

DD 250,850 also describes an angle-resolving method of measuringdiffused light by means of fiber-optic light guides arranged in acircle.

Both the above methods have the drawback that the collector angle ismuch smaller and closer to specular than that described in the presentdisclosure.

In this connection, U.S. Pat. No. 4,314,763 describes a design in whichperpendicular incident light and rotational symmetry of the collectoroptics about the perpendicular of the test object permit measurementsregardless of the defect's orientation. But the lens system used onlyhas a small collector angle and this limits the capability of the designin detection of very small particles at a high throughput rate.

The same inventor's U.S. Pat. No. 4,598,997 improves the measurement oftextured or structured surfaces by the addition of a special mask to thedesign described above. The purpose of the mask is to suppress the raysdeflected by these structures.

A significant drawback of prior art systems is the inability to detectvery small surface or near surface defects and particles. With thecontinual reduction in size of semiconductor structures on wafersurfaces, it is critically important to be able to detect such smallanomalies. As shown in Table 34 of The National Technology Roadmap forSemiconductors by The Semiconductor Industry Association, 1994, therequirements for defect and particle detection sensitivity will be 0.08micron in 1998, 0.05 or 0.06 micron in 2001 and down to 0.02 micron in2010. None of the above referenced systems is capable of achievingsensitivities that are close to such requirements.

SUMMARY OF THE INVENTION

As noted above, it is difficult for prior art systems to detect smallanomalies such as small particles and defects. Small particles ordefects scatter light at large angles to the normal direction of thesurface when the surface is illuminated in the normal direction. Innormal illumination prior art systems where the light scattered by thesurface is collected by a lens system where the axis of the lens systemis along the normal direction, the lens system will collect only a smallportion of the light scattered by such small anomalies. If largeanomalies such as particles or surface defects are also present inaddition to the small anomalies, the scattering from such largeanomalies will be at much higher intensities compared to and will maskthose caused by the small anomalies so that the small anomalies becomedifficult or impossible to detect. One aspect of the invention is basedon the recognition that, since the scattering from the large anomaliesis at much higher intensities at specular or near specular collectionangles (that is, small angles to the normal) than at large collectionangles whereas the scattering from small anomalies have intensitieswhich are more evenly distributed in all directions to the normal, withmost of the energy contained in the larger angles. The detection of thesmall anomalies can therefore be much enhanced by using an ellipsoidalmirrored surface to collect light scattered at relatively largecollection angles to the normal and avoiding light scattered at specularor near specular directions.

Thus, one aspect of the invention is directed towards an optical systemfor detecting contaminates and defects on a test surface comprising asource of light to produce a beam, means for directing the beam along apath onto the test surface, producing an illuminated spot thereon. Thesystem further includes an ellipsoidal mirrored surface having an axisof symmetry substantially coaxial with the path, defining an inputaperture positioned proximate to the test surface to receive scatteredlight therethrough from the surface. The mirrored surface reflects andfocuses light that is rotationally symmetric about said axis of symmetryand that passes through the input aperture at an area. The systemfurther includes means for detecting light focused to the area.

When it is known that the surface scattering or haze level is low, andthat there are few large defects or point-anomalies, detectionsensitivity for small anomalies can be further enhanced by adding to theellipsoidal mirrored surface of the above apparatus a lens assembly thatcollects light scattered in a small angle region near the speculardirection and focuses the collected light to the same area as theellipsoidal mirrored surface.

Another aspect of the invention is based on the observation that largerparticles scatter light at smaller angles to the normal direction of thesurface (i.e. direction of the specularly reflected beam) compared tosmaller particles, and the light scattered by the smaller particles islower in intensity compared to the light scattered by larger particlesor defects. Where light scattered in a range of angles covering thecollection angles for both large and small particles is collected anddirected to a single detector means, and if the detector means isoptimized for detecting the low intensity light scattered by smallerparticles, the detector means may become saturated by the high intensitylight scattered by larger particles. On the other hand, if the detectormeans is optimized for detecting the high intensity light scattered bylarger particles, it is not optimized to detect low intensity lightscattered by the smaller particles.

Furthermore, the surface texture itself produces a certain amount ofdiffracted light in addition to the light scattered by particles. Thissurface light scatter, commonly referred to as haze, tends to beconcentrated at smaller angles near the specularly reflected light beam.If a single detector arrangement is used to detect scattered light fromboth large and small particles or defects, the effect of haze is tosignificantly degrade the signal-to-noise ratio for the detection of thesmaller defects and particles.

This aspect of the invention is based on the observation that, bycollecting scattered light in directions close to and at smaller anglesto the specular reflection direction separately from light scattered atlarger angles to the specular reflection direction and directing thelight scattered at smaller angles to a different detector than the lightscattered at larger angles, it is now possible to optimize the two ormore detectors separately. Thus, two or more detectors are used: atleast a first detector for detecting the low intensity light scatteredby smaller particles at larger angles to the specular reflectiondirection, and at least a second detector for detecting the highintensity light scattered by larger particles at smaller angles to thespecular reflection direction. The first detector will not be seriouslyaffected by scattering due to haze, since such scattering decreasesrapidly at larger angles from the specular reflection direction.

The above concept is applicable even where the light beam forilluminating the surface to be inspected is at an oblique angle to thesurface instead of being perpendicular to the surface and is alsoapplicable for the differentiation, characterization and/orclassification of different types of surface or near surface anomalies(referred to below simply as anomalies of surfaces or surfaceanomalies), including but not limited to anomalies such as scratches,slip lines, crystal originated particles (COPs) as well as contaminationparticles.

As indicated above, the requirements for detection sensitivity arebecoming more and more stringent. For such purpose, it is desirable tofocus the illuminating beam onto a small spot on the inspected surface,such as one no larger than 50 microns in dimensions in any direction onthe surface. This will enhance signal-to-noise ratio.

Thus, another aspect of the invention is directed towards an apparatusfor detecting anomalies of surfaces, comprising means for focusing alight beam along a path towards a spot on a surface, causing a specularreflection, said spot having dimensions less than 50 microns; means forcausing rotational and translational movement of the surface, so thatthe beam scans the surface along a spiral path. The apparatus furthercomprises a first detector located to detect light scattered by thesurface within a first range of collection angles and a second detectorlocated to detect light scattered by the surface within a second rangeof collection angles, said second range being different from the firstrange; and an ellipsoidal mirrored surface defining an input aperturepositioned approximate to the surface to receive scattered lighttherethrough from the surface, the mirrored surface reflecting andfocusing light passing through the input aperture at the first detector;and a lens assembly collecting light passing through the input aperture,defining collected light, said lens assembly focusing the collectedlight to the second detector.

Yet another aspect of the invention is based on the observation that ifthe lens used for collecting light to a detector is also used to focusthe illuminating beam towards the surface inspected, stray reflectionsand scatter of the illuminating beam at the collection lens can causesuch background light to be detected by the detector. This introduceserrors and is undesirable. Thus, another aspect of the invention isdirected towards an apparatus for detecting anomalies of surfaces,comprising means for directing a light beam towards a surface in adirection substantially normal to the surface, said direction definingan axis; means for causing relative motion between the surface and thebeam, so that the beam scans the surface; and means for detecting lightscattered by said surface. The detecting means includes at least onelens for collecting light to be detected, wherein the directing meansdirects light towards the surface along an illumination path that doesnot pass through said at least one lens. The detecting beam preferablyincludes at least two detectors: a first detector located to detectlight scattered by the surface within the first range of collectionangles from the axis and a second detector located to detect lightscattered by the surface within a second range of collection angles fromthe axis, said second range being different from the first range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a wafer-inspection assembly, with twowafer cassettes and an automatic wafer-transport and wafer-measuringdevice.

FIG. 2 is a diagrammatic representation of the present state of the art.

FIG. 3 is a further diagrammatic representation of prior art, but fittedwith an assembly as described in the present disclosure.

FIG. 4 shows a first embodiment of an assembly as described in thepresent disclosure.

FIG. 5A is a graphical illustration of the scattered light intensityfrom PSL spheres of different diameters at different angles ofcollection from the normal direction of the surface.

FIG. 5B is a graphical illustration of the surface scattering backgroundintensities of silicon at different angles of collection from the normaldirection of the surface.

FIG. 5C is a graphical illustration of the signal-to-noise ratio of PSLspheres on silicon at different angles of collection from the normaldirection.

FIG. 6 shows a second embodiment of the assembly as described in thepresent disclosure.

FIG. 7 is a schematic view of a surface inspection system to illustratethe preferred embodiment of the invention.

FIG. 8 is a graphical plot of the scattered light intensity from asilicon surface, and from a large and a small PSL sphere placed on thesurface to illustrate the invention.

FIG. 9 is a schematic view of a surface inspection system to illustratean alternative embodiment of the invention.

For simplicity in description, identical components are identified bythe same numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The purpose of the present invention is, therefore, to propose a methodand an assembly that avoids the drawbacks of prior art as describedabove and whose measuring sensitivity for particles and defects on testobjects subjected to inspection is substantially greater, but withoutlimiting haze sensitivity. At the same time, it should provide simplemeans to permit the adjustment of the sensitivity of the assembly, thesize of the illuminated spot on the test object, and thus of the amountof diffused light generated by point defects. In addition, it shouldmake available as large an unmodified portion of the diffused light aspossible, to provide a flexible means of selectively separating andfurther processing the relevant parts of the diffused light, to suit theparticular inspection task at hand.

The present disclosure achieves these aims by the characteristicfeatures described below which provide the main advantages of theinvention, namely:

-   -   the spatially stationary arrangement of projection and collector        optics ensures that when the test object is moved parallel to        its surface the measuring sensitivity remains constant over the        whole surface of the object;    -   because the incident beam and collector optics are rotationally        symmetrical about a line perpendicular to the test object's        surface, the orientation of a defect such as polishing scratch        does not affect measuring sensitivity;    -   use of a rotationally symmetrical ellipsoidal mirror creates a        very large area for the collection of diffused light, and this        is very important for the detection of particles in the 100 nm        range and below, that diffuse fairly evenly throughout the        hemisphere;    -   an appropriate lens system that allows light collection in the        central zones near the axis is particularly important for the        detection of particles in the 1 μm and above range, because of        the extensive amount of scattering of such particles at smaller        angles to the normal direction of the surface; and    -   projection on a collection diaphragm of the image of the light        diffused by the illuminated spot provides great design for a        large variety of aperture configurations and makes possible        positionally resolved, angle resolved, polarizing, and other        measurements, without thereby affecting the illuminating beam or        requiring any adjustments to the central part of the assembly,        comprised of an ellipsoidal mirror, a lens system, a beam        deflector, and a dark field stop. This is crucially important in        view of the fact that the specified quality of these components        is very high and precise dimensioning is extremely delicate. For        example, the haze level for top-quality wafers is about 50 ppb        (parts per billion), hence only about 0.000,000,05 of the total        amount of light introduced is directed to the photosensor.        Because haze interferes with the measurement of the smallest        particles, it is essential to keep it to a minimum. To prevent        measurements being affected (impaired), this also means that        parasitic and ambient light from optical components must be much        less than 50 ppb.

The assembly can be further improved by the use of different lenscombinations in the projection optics to project the image of theilluminated spot to suit the test object, thus providing greaterflexibility as to the size and shape of the illuminated spot. This alsomakes it possible to adjust the measuring sensitivity to different typesand shapes of particles and surface defects that depend on theilluminating spot profile.

In a useful further development of the invention described in thepresent disclosure by a different configuration of the collectiondiaphragm or of the light guides above the collection diaphragm:

-   -   the signal-to-noise ratio can be improved for particle        measurement by the selective adjustment of the optimum        collection angle;    -   defects with certain diffusion properties can be either selected        or suppressed;    -   angle-resolved diffused-light measurement for the known        light-reflection behavior of different types of defects can be        used for defect classification; and    -   certain spatial frequencies of the surface can be separated.

In yet another configuration, variable attenuators are fitted above thecollection diaphragm to permit the photodetector to operate in itsoptimum working range. In other words, where the reflection orscattering from the surface is at high intensity, such as where thesurface is metallized, the attenuator will reduce the intensity to avalue within the detector optimum working range. Where the reflection orscattering from the surface is not at high intensity, attenuationapplied by the attenuator may be reduced or disabled so that theintensity detected is at a value within the detector optimum workingrange. This increases the dynamic range of the detector for detecting awide range of intensities from a wide variety of surfaces. These are asimple means for reducing the amount of light, but do not affect theprojection or collector beam.

FIG. 1 shows a substrate-surface inspection system as used mainly forthe inspection of wafers. Such systems are used to display the presenceof extremely small particles (i.e. about 100 nm in size), crystaldefects, metallic impurities, polishing defects, scratches, and ofimplant and other inhomogeneities on wafers.

One condition for the measurement of very small particles in the 80 nmrange is that it must be performed in a very clean environment, such asa class 1 clean room.

A state-of-the-art means of ensuring the requisite cleanliness for suchmeasurements is, for example, the use of a flow box and anaerodynamically transparent design.

FIG. 2 is a diagrammatic representation of a state-of-the-art surfaceinspection system based on the principle of measuring elastic diffusion.A light source 1, usually a laser beam, illuminates a dot-shaped point 2on the surface of a wafer 3. The specular reflected light leaves thesystem in the direction of arrow BF. A first lens 5 collects part 4 ofthe light diffused by the surface and projects its image to aphotodetector 7. An output signal 8 from the photodetector 7 isconducted to an amplifier 9. The wafer surface 3 subjected to inspectionlies in the so-called focal plane 10. If there is a defect at theilluminated position 2, the amount of diffused light 4 increases, asdoes the intensity of the light reaching the photodetector 7, and hencethe output voltage (U_(a)) 11 from the amplifier 9 also rises.

FIG. 3 shows an assembly of parts of a surface-inspection system. Thelight emitted by a laser 20 passes through an optical filter 21, forexample an attenuator or neutral density filter, to a beam deflector 22,such as a mirror or prism, and thence to a lens 23 which focuses thelight to an illuminated spot 24 lying in the focal plane 10. During theinspection procedure the wafer surface 3 subject to inspection lies inthe focal plane 10. That portion of the diffused light 4 which isdiffused by the wafer surface 3 passes through the collector lens 23 andthe collection diaphragm 6 to the photodetector 7. In this confocalsystem the aperture 25 in the collection diaphragm 6 lies in the image26 of the illuminated spot 24 and its shape is more or less the same asthat of the spot 24. In the calibration phase, a reference medium 27 isplaced in position, preferably below the focusing plane 10.

Because the reference medium 27 thus lies beyond the focusing plane 10,the area of a second illuminated spot 28 is greater than the first suchspot 24 in the focusing plane 10. Likewise, in the calibration phase,the area 29 thus illuminated in the position of the collection diaphragm6 is greater than the image 26.

Because the aperture 25 of the collection diaphragm 6 is still in thesame size, only a very small proportion of the diffused light from thereference medium 27 now passes through the aperture 25 of the collectiondiaphragm 6 to the photodetector 7.

An attenuation mechanism for the calibration is formed by means of theoptical filter 21 and/or by displacement of the reference medium 27 outof the focusing plane 10.

When the position of the reference medium 27 is moved along the opticalaxis 32, this alters the size of the illuminated area 29 in the positionof the collection diaphragm 6.

Thus, if the size of the aperture 25 in the collection diaphragm 6 iskept constant, this makes it possible to regulate the amount of energythat reaches the photodetector 7.

The typical embodiment shown has an adjustment mechanism 37 whichconsists of a support 34 adjustable in height by means of an adjustmentscrew 33, on which the reference medium 27, having surface 30 and volume31, is placed. The support for the reference medium has a raised rim 38.

In this typical embodiment the height setting can be fixed by means of aclamping screw 35.

Further, it is of course perfectly feasible to integrate at least onerefracting element 36, for example a lens, between the light source 20and the beam deflector 22.

The lenses, laser light sources, diaphragms, etc., described in FIGS. 3,4, and 6 can of course be assembled as complete systems, and in practicethis is the case. Thus, for example, the light emitted by the lightsource 20 may be coherent or incoherent, monochromatic or polychromatic,unpolarized or polarized, and elliptical, linear, or circular, and mayemanate from one or two lasers of different wavelengths, a mercury-vaporlamp etc. The lens may be a single spherical or cylindrical lens, or acomplete lens system. Further, to facilitate adjustments to the opticalsystem, additional mirrors may be placed between the light source 20 andthe deflection mirror 22.

For the sake of clarity, the drawings omit these details which may, forexample, be necessary for adjustment and/or calibration.

For the purposes of the present disclosure it is also assumed that theprocess and assembly described in the present disclosure are applied tothe prior art, in particular to U.S. Pat. No. 4,314,763 (Steigmeier etal.), wherein the transport system for spiral scanning used to scan thewafer subjected to inspection makes a composite movement consisting oftranslating and rotation, though the principle as such must be regardedas known.

Moreover, the present disclosure includes a gauge, described below,which is essential to ensure that the light supplied by the light sourceis perpendicular to the test object's surface and that the light sourceand the supply of light remain stationary while the test object movesspirally under the light beam during the scanning process, maintainingrotational symmetry.

FIG. 4 shows a first such typical embodiment of a system as describedfor the invention in the present disclosure. For clarity, this adoptsthe same reference numbers as those used in FIG. 3 for all features thatoccur in both figures.

As shown, the light emitted by the source 20 passes through a projectionlens 36′, via the beam deflector 22, to the illuminated spot 24 on thetest object (wafer) 3. In this case, the size and shape of theilluminated spot 24 are determined and adjusted solely by the imageproduced by the projection lens 36′ system. The light L_(O) directlyreflected by the wafer 3 passes along the same path back to the lightsource, and a dark-field stop 41 helps ensure that the directlyreflected near specular light L_(O) does not reach the photodetector 7.

Any surface inhomogeneities that may be present on the wafer 3 subjectedto inspection diffuse the incident light throughout the hemisphere abovethe illuminated spot 24. An ellipsoidal mirror or mirrored surface 42 isprovided to ensure that the maximum amount of the diffused light istransmitted to the photodetector 7; the mirror 42 is fitted rotationallysymmetrically about the optical axis above the illuminated spot 24 andbelow the beam deflector 22.

The internally silvered or aluminized ellipsoidal mirror 42 is shaped asa partial rotation ellipsoid. The beams of diffused light L₁ and L₂ andall the rotationally symmetrical beams thus collected by the ellipsoidalmirror 42 form the image of the illuminated spot 24 on the aperture 25of the collection diaphragm 6.

In this system, the collection diaphragm 6 has the task, on the onehand, of preventing unwanted diffused light that may, for example, beproduced in the optical components, from reaching the photodetector 7,and, on the other, of allowing the beams of diffused light L₁ and L₂from the illuminated spot 24 to pass.

The advantages of the optical inspection system of FIG. 4 will now bedescribed in reference to the scattering characteristics of smallanomalies illustrated in FIGS. 5A-5C. FIG. 5A is a graphicalillustration of the scattering light intensities from PSL spheres ofdifferent diameters collected at different angles of collection from thenormal direction to the surface. As shown in FIG. 5A, for the same sizePSL sphere, the intensity of scattered light at a smaller collectionangle to the normal is smaller than that at a larger angle ofcollection. Sensitivity of detection is the ability to differentiate asignal originating from an anomaly from that originating frombackground. Therefore, in addition to accounting for the strength of thelight signal from the anomaly, the strength of the background signalwill also have to be taken into account; this is illustrated in FIG. 5B.As shown in FIG. 5B, it is clear that the scattering backgroundintensity of silicon is much stronger at near specular collection anglesin the range of 2 to 5° as compared to that at large collection anglesto the normal such as 65 to 85° or 25 to 65°. FIG. 5C is a graphicalplot of the signal-to-noise ratio as a function of sphere diameter forthe four different ranges of collection angles. From FIG. 5C, it isclear that the signal-to-noise ratio at large collection angles is muchbetter for small particles compared to that at near specular or smallcollection angles.

The optical system of FIG. 4 includes an ellipsoidal mirror 42 shaped tocollect light at large collection angles to the normal direction andavoids collecting light at near specular or small collection angledirections. Thus, from FIG. 5A it is evident that at the same collectionangle, the scattering intensity from a large particle is generallyhigher than that from a smaller particle. It is also further observedthat at certain particle size, there will be essentially near zerointensity of scattered light in the range of near specular directionswhereas there may still be detectable scattering intensity at largercollection angles. For example, if the scattering intensity from 0 to 5°is avoided altogether, particles smaller than 100 nanometers are stilldetectable at collection angles of 3 to 25°, 25 to 65° and 65 to 85°.But if the same detector used to detect smaller particles also detectslight in the near specular region, detection of light scattered by suchsmall particles will be made difficult by light from the largerparticles, and by the surface background as well. At a sphere size ofabout 100 nanometers, it appears that such smaller spheres aredetectable only within the ranges of 3 to 25° and 25 to 65° so that iflight is collected only within such ranges, detection of light from suchtiny particles will not be made difficult by light from the largerparticles and are detectable. However, very large surface scattering(haze) levels will mask the detection of small particles within thecollection angles 3 to 25°. The system of FIG. 4 is thereforeadvantageous for detection of small particles and defects since itcollects only light within one or more ranges of the larger collectionangles such as 25 to 65°, 65 to 85° and not 2 to 5° or even 3 to 25°from the normal.

FIG. 6 shows the function of two lenses 39 and 40 in connection with theellipsoidal mirror 42 and the separate rays L₃, L₄, L₅, and L₆ thatindicate beams of diffused light. The configuration in FIG. 6 may beuseful where the number of larger particles or defects on thesemiconductor wafer surface is insignificant compared to the smallparticles or defects and also for very low background surfaces. In suchcircumstances, it may be advantageous to also collect light in the rangeof small collection angles 3 to 25° from the normal direction of thesurface by means of one or more lenses. However, to include collectionin the near specular region (2 to 5°) is undesirable.

The important new features in FIG. 6 are two lenses 39, 40 in theoptical path between the beam deflector 22 and the illuminated spot 24,i.e. a first lens 39 and a second lens 40 placed on the optical axis,for the purpose of collecting as much light as possible and focus thelight to the same area as the mirror 42. This is exactly the samepurpose as that pursued and achieved by the use of an ellipsoidal mirror42 which is part of a rotation ellipsoid and whose axis of symmetry isparallel to the optical axis, and where the two focal points of theellipsoid lie, on the one hand, in the illuminated spot 24 and, on theother, in the image 26.

The use of the two lenses 39 and 40 in conjunction with the ellipsoidalmirror 42 increases the collection area.

Two lenses are necessary to prevent the rays from the ellipsoidal mirrorstriking the focusing unit located on the optical axis and to maximizethe area between the beam deflector 22 and the illuminated spot 24. Inother words, the use of two lenses enables more light to be collectedwhile retaining the function of focusing the incoming illuminating beamonto spot 24 and the outgoing scattered light onto image 26. Theposition and focal length of these lenses must be so chosen as to ensurethat the focusing unit on the optical axis also forms an image of thelight spot on the collection diaphragm 6 in front of the photodetector7. As described above in connection with FIG. 4, unless it has alreadybeen integrated in the beam deflector, the dark-field stop 41 preventsdirectly reflected near specular laser light and any light diffused byoptical components from reaching the photodetector 7.

With the introduction of locally resolved measurements, for example bythe use of detector arrays instead of a simple photodetector 7, thesignal-to-noise ratio can be further improved, because the effect ofhaze is equally powerful on all the detectors but the light-point defect(LPD) produces a greater response in some detectors than in others.

To ensure that the photodetector operates in its optimum working rangewhen the substrate subject to inspection produces substantial diffusion,it may be necessary to use attenuators 79 between diaphragm 6 andphotodetector 7 to increase the dynamic range of the photodetector.

As already described, the use of combined spherical and cylindricallenses 36′, instead of a simple cylindrical lens 36 in astate-of-the-art assembly, can be useful to adjust the size and shape ofthe light beam projected to the illuminated spot 24.

Because the brightness of the reflected light that appears in the image26 is increased by the means described above, angle-resolved measurementalso becomes possible, for example by the use of light guides fittedbetween the collection diaphragm 6 and the photosensor 7, in order toeliminate certain angles of diffusion.

Also to eliminate rays at certain angles of diffusion, the configurationcan be further altered by the provision of a second diaphragm 6′ (notshown) above or below the collection diaphragm 6.

As noted above, it may be desirable to collect and direct at a detector,light at large collection angles without mixing such collected lightwith light within small collection angles or near specular reflectionfor the purpose of detecting tiny particles or defects. Where detectionof larger particles and defects is also desired, it may be advantageousto use a second detector to detect light within small collection angles(e.g. 3 to 25 degrees) to the normal in the manner shown in FIG. 7.

As shown in FIG. 7, the surface inspection system 1010 may be used forinspecting anomalies on a surface 1012. Surface 1012 is illuminated by asubstantially stationary illumination device portion of system 1010comprising a laser beam from a laser source (not shown). The laser beam1014 is passed through polarizing optics 1016 of the device portion tocause the laser beam to have the desired polarization state when used toilluminate surface 1012. Laser beam 1014 is then passed through a beamexpander and aperture 1018 and beam-forming optics 1020 to expand andfocus the beam 1014′. The beam 1014′ is then reflected by a beam foldingcomponent 1022 and a beam deflector 1024 to direct the beam 1014″towards surface 1012 for illuminating the surface. In the preferredembodiment, beam 1014″ is substantially normal or perpendicular tosurface 1012, it being understood that this is not required and many ofthe advantages of the invention described herein are equally applicablewhere beam 1014″ is at an oblique angle to surface 1012.

In the preferred embodiment, beam 1014″ is substantially perpendicularor normal to surface 1012 and beam deflector 1024 reflects the specularreflection of the beam from surface 1012 towards component 1022, therebyacting as a shield to prevent the specular reflection from reaching thedetectors. The direction of the specular reflection is along line SRnormal to surface 1012. In the preferred embodiment where beam 1014″ isnormal to surface 1012, this line SR coincides with the direction ofilluminating beam 1014″, where this common reference line or directionis referred to herein as the axis of system 1010. Where beam 1014″ is atan oblique angle to surface 1012, the direction of specular reflectionSR would not coincide with the incoming direction of beam 1014″; in suchinstance, the line SR indicating the direction of the surface normal isreferred to as the principal axis of the collection portion of system1010.

Light scattered by small particles are collected by mirror 1038 anddirected towards aperture 1040 and detector 1044. Light scattered bylarge particles are collected by lenses 1032 and directed towardsaperture 1036 and detector 1042. Large particles will also, of course,scatter light that is collected and directed to detector 1044, and smallparticles will also scatter light that is collected and directed todetector 1042 but such light is of relatively low intensity compared tothe intensity of scattered light the respective detector is designed todetect.

To illustrate the preferred embodiment of the invention, FIG. 8 showsgraphical plots of the scattered light intensity (1050) from a siliconsurface, that (1054) from a small PSL sphere of the order of 100nanometers (nm) diameter placed on the surface and that (1052) from alarge PSL sphere of the order of 1 micron diameter placed on thesurface. In reference to FIG. 7, the polar angle of FIG. 8 indicates thecollection angle of the scattered light away from the axis SR of system1010. Thus, the intensity at a polar angle of zero degrees wouldindicate the intensity of light reflected or scattered by surface 1012or the PSL sphere along the axis SR of system 1010 as shown in FIG. 7.As shown by curve 1050, in FIG. 8, the light scattered by the siliconsurface falls off rapidly away from the polar angle zero, where specularreflection occurs. The light scattered by the silicon surface away fromthe specular reflection direction is frequently due to haze; as shown inFIG. 8, light scattering due to haze falls off rapidly with increasingcollection angles to the axis of the system. Specular reflection as wellas scattered light at collection angles up to about 5° are deflected bydeflector 1024 and does not reach any one of the two detectors 1042 or1044. Light scattered at collection angles within the range of 5-20°from the axis SR of system 1010 are collected by lenses 1032 anddeflected by beam deflector 1034 towards an aperture 1036, so that theportion of the beam that passes aperture 1036 is detected by detector1042. Light scattered at collection angles in the range of about 25 toabout 70 degrees are collected by mirror 1038 and focused towards anaperture 1040 so that the light that passes through the aperture isdetected by detector 1044.

The angular distribution of light scattered by the small size PSL sphereis shown as the solid line curve 1054 in FIG. 8. As shown in FIG. 8,small particles preferentially scatter at higher angles than a siliconsurface. It is also known that small particles scatter at higher anglesthan larger particles. Whereas the intensity of scattering peaks ataround 30-40° for a 100 nanometer PSL sphere, the scattered lightintensity typically peaks at much lower scattering angles for large sizespheres (about 1 micron diameter and greater). See curve 1052 in FIG. 8.The device portion of system 1010 for collecting and detecting scatteredlight from anomalies such as large particles is comprised of lenses1032, a folding mirror 1034, aperture 1036, and detector 1042. Mirror1038, aperture 1040, and detector 1044 are adapted to detect scatteredlight from smaller particles, and form the device portion of system 1010for collecting and detecting scattered light from anomalies such assmall particles and defects. Since larger particles typically scatterlight at higher intensities compared to smaller particles, the detectors1042, 1044 can be optimized separately, with detector 1042 optimized fordetecting large particles and detector 1044 optimized to detect smallerparticles. By using two different detectors for detecting scatteredlight within two different ranges of collection angles, each detectorcan be optimized for detecting the respective types of particles and theuser is not forced to choose optimization for detecting one type ofparticle versus the other. Instead both detectors can be optimized todetect their respective types of particles simultaneously.

The meaning of “large” and “small” anomalies discussed above may bephrased in more general terms. In general, an anomaly is small if itsdimensions are a fraction of the wavelength of the electromagnetic(laser) radiation used to illuminate the surface inspected. Thus, theplot of FIG. 8 shows the scattering from PSL spheres that are “large”and “small” with respect to visible light wavelengths. If radiation ofother wavelengths are used, then the meaning of “large” and “small”anomalies will change according to such wavelengths.

If a single detector or detector arrangement is chosen to detect thelight scattering from both large and small particles, a largerdark-field stop must be employed to prevent near angle (near specular)surface scatter from surface 1012 from reaching the detector. This wouldbe necessary in order to maintain the sensitivity of the detector to lowintensity scattering from smaller particles. A larger aperture stopwould therefore decrease the sensitivity of the system towards lightscattering by large particles and also to surface scatteringcharacteristics at near specular angles of collection. This isundesirable. System 1010 of FIG. 7 avoids such undesirable compromise.Since separate detectors 1042, 1044 are now employed, the design of bothlight collection and detection subsystems need not be constrained sothat the range of collection angles for lenses 1032 may be increased toinclude the near specular collection angles as well. While preferablylenses 1032 collect light that are scattered in a range of 5-20°, suchrange may be extended to, for example, 3-25°. The larger ranges ofcollection angles would be useful for particle and defectcharacterization/classification or surface topography, in someapplications.

As shown in FIG. 8, light scattering caused by haze falls off rapidlywith increasing collection angles, so that there is negligible lightscattering caused by haze that is collected by mirror 1038 and directedtowards detector 1044. This further enhances the sensitivity andaccuracy of system 1010 for detecting smaller anomalies. In thepreferred embodiment, mirror 1038 collects and focuses scattered lightin the range of 25-70° from the axis of system 1010 towards aperture1040 and detector 1044. As indicated above, detectors 1042, 1044 may beoptimized separately to have different intensity detection thresholds.

From the above description, it is seen that beam deflector 1024 serves adual function: to deflect the illuminating beam so as to provide beam1014″ and also acting as a stop to shield detectors 1042, 1044 fromspecular and near specular (or semi-specular) diffuse reflection. Itshould also be noted that the illumination portion and the detectiondevice portions of system 1010 are designed so that the illuminationbeam, in its entire path from the laser source until surface 1012, doesnot pass through any lens or lens arrangement of the detection system.In the preferred embodiment shown in FIG. 7, this is implemented byplacing the beam deflector 1024 between lenses 1032 and surface 1012. Aninput aperture 1038 a in mirror 1038 permits the illuminating laser beamto be passed from beam turning component 1022 to beam deflector 1024 soas to enable beam deflector 1024 to be placed between lenses 1032 andsurface 1012. Mirror 1038 is preferably ellipsoidal in shape and alsopreferably substantially rotationally symmetrical about axis SR ofsystem 1010, so that the same detection result can be obtainedrepeatedly irrespective of the relative orientation of surface 1012 andof any defects thereon with respect to the illumination and thedetection device portions of system 1010. Thus the light detected bydetectors 1042, 1044 within the two ranges of collection angles issubstantially rotationally symmetrical about the axis of system 1010upon such light scattering by surface 1012.

FIG. 9 is a schematic view of a surface inspection system 1100 toillustrate an alternative embodiment of the invention. The system 1100is similar to system 1010 of FIG. 6 except that the bottom portion ofmirror 1038′ has a different curvature than the remaining portion, sothat where the remaining portion focuses the light scattered by surface1012 towards aperture 1040 and detector 1044, portion 1038 b has adifferent curvature so that portion 1038 b together with a beam turningcomponent 1102 focus light scattered by surface 1012 towards an aperture1104 and detector 1106 for detection. Preferably, portion 1038 b isparaboloid in shape and it collimates the light scattered from surface1012, where the collimated light is focused by a curved mirror 1102towards aperture 1104 and detector 1106. Alternatively, portion 1038 bmay have a focal point and focuses the scattered light impinging on ittowards beam turning component 1102 that reflects such light towards anaperture 1104 and detector 1106 for detection. Portion 1038 b andcomponent 1102 preferably collect and focus light in the range of about65 to 85 degrees from axis SR towards aperture 1104 and detector 1106.The remaining portion of mirror 1038′ collects and focuses light in therange of about 25 to 60 degrees from axis SR towards aperture 1040′ anddetector 1044. The lenses 1032′ collects and focuses light in the rangeof about 5 to 20 degrees (or even 3 to 25 degrees) from axis SR towardsaperture 1036′ and detector 1042. The use of three sets of lightcollection optics and detectors to separately detect the scattered lightin smaller ranges of angles from axis SR may be advantageous for someapplications. Obviously, mirror 1038′ may have more than two portionshaving different curvatures, in order to separately detect the scatteredlight in more than three smaller ranges of angles from axis SR. Such andother variations are within the scope of the invention.

Rotational and translational movement of surface 1012 is caused in aconventional manner so that beam 1014″ scans the surface along a spiralpath. Thus, as shown in FIG. 9, the semiconductor wafer 1011 havingsurface 1012 thereon may be supported by a supporting disk 1072 which isconnected to a shaft 1112 having axis 1074 of a rotary motor 1114 whichis in turn fixed to a linear translation stage 1116, driven by atranslation motor 1118. The rotary and translation motors are controlledin a coordinated manner as known to those skilled in the art to causesimultaneous rotational and translational movement of surface 1012 sothat beam 1014″ would trace a spiral path on surface 1012. Surface 1012of FIG. 7 may be caused to travel in the same manner so that beam 1014″scans surface 1012 along a spiral path.

In operation, a light beam such as beam 1014″ is directed towardssurface 1012 in a specified direction or angle of incidence, causingspecular reflection along a direction defining an axis. Rotational andtranslational movement of the surface is caused so that the beam scansthe surface along a spiral path. Light scattered by the surface withinthe first range of collection angles from the axis is detected by meansof a first detector. Light scattered by the surface within the secondrange of collection angles from the axis is detected by means of asecond detector, where the two ranges of collection angles aredifferent. Preferably, the two ranges of collection angles aresubstantially stationary. Preferably, the axis is substantially normalto the surface.

While the invention has been described above by reference to thepreferred embodiment, it will be understood that various changes andmodifications may be made without departing from the scope of theinvention which is to be defined only by the appended claims. Forexample, while only one detector has been shown for each of the twodetectors 1042, 1044, it will be understood that an array of detectorsmay be used for each of the two detector locations 1042, 1044.Additional apertures or aperture stops may be employed in the detectionand illumination portions of system 1010 than as shown in FIG. 7. Theillumination beam and the collector light may also be passed throughmore or fewer lenses or mirrors of different optical arrangements thanas shown in FIG. 7. All such variations are within the scope of theinvention. The system described is also advantageous for differentiatingbetween scratches, slip lines, COPs and other topographic features,since one type of such defects may scatter light at a larger angle tothe axis compared to another type of such defects.

1. An optical system for detecting contaminants and defects of a testsurface comprising: a device providing a polarized light beam along apath at an oblique angle to the test surface, producing an illuminatedspot thereon; a first and a second detector; a first collector having anoptical axis substantially along a line perpendicular to the testsurface, said first collector collecting light from the polarized lightbeam scattered by the illuminated spot of the surface and conveying thecollected light to the first detector, causing the first detector toprovide a single output in response to the light collected by the firstcollector; and a second collector collecting light from the polarizedlight beam scattered by the illuminated spot of the surface andconveying the collected light to the second detector, causing the secondcollector to provide a single output in response to the light collectedby the second collector, wherein the first and second collectors collectlight scattered by the surface within different ranges of collectionangles from the line.
 2. An optical system for detecting contaminantsand defects of a test surface comprising: a device providing a polarizedlight beam along a path at an oblique angle to the test surface,producing an illuminated spot thereon; a first and a second detector; afirst collector collecting light scattered by the surface and conveyingthe collected light to the first detector; and a second collectorcollecting light scattered by the surface and conveying the collectedlight to the second detector, wherein the first and second collectorshave an optical axes substantially along a line, said first and secondcollector collecting light scattered by the surface within differentranges of collection angles that are away from and that do not include aline normal to the test surface, said first collector having an opticalaxis substantially along a line perpendicular to the test surface. 3.The system of claim 2, wherein the first collector collects lightscattered from the spot within collection angles of up to 25 degreesfrom the line.
 4. The system of claim 2, wherein the first collectorcollects light scattered from the spot within collection angles of about3 to 25 degrees from the line.
 5. The system of claim 2, wherein thesecond collector collects light scattered from the spot withincollection angles of about 25 to 70 degrees.
 6. The system of claim 2,said system further comprising a third detector, said three detectorslocated to detect light scattered by the surface within at least afirst, second and third range of collection angles from the line, saidfirst range of angles being about 3 to 25 degrees, and said second rangebeing about 25 to 65 degrees, and said third range being about 65 to 85degrees.
 7. The system of claim 2, said system further comprising aninstrument causing relative rotational and translational motion betweenthe beam and the surface, so that the beam scans a spiral path on thesurface.
 8. The optical system of claim 2, wherein the device provides alinearly polarized beam.
 9. An optical system for detecting contaminantsand defects of a test surface comprising: a device providing a polarizedlight beam along a path at an oblique angle to the test surface,producing an illuminated spot thereon; a first and a second detector; afirst collector collecting light scattered by the surface within a firstrange of collection angles, said first collector conveying the collectedlight to the first detector; and a second collector collecting lightscattered by the surface and conveying the collected light to the seconddetector, wherein the second collector collects light scattered by thesurface within a second range of collection angles that are differentfrom the first range of collection angles, at least one of said firstand second ranges of collection angles being away from and do(es) notinclude any direction normal to the test surface, and wherein said firstand second ranges of collection angles do not include any specularreflection direction of the light beam, said first collector having anoptical axis substantially along a line perpendicular to the testsurface.
 10. The system of claim 9, wherein the first collector collectslight scattered from the spot within collection angles of up to 25degrees from the line.
 11. The system of claim 9, wherein the firstcollector collects light scattered from the spot within collectionangles of about 3 to 25 degrees from the line.
 12. The system of claim9, wherein the second collector collects light scattered from the spotwithin collection angles of about 25 to 70 degrees.
 13. The system ofclaim 9, said system further comprising a third detector, said threedetectors located to detect light scattered by the surface within atleast a first, second and third range of collection angles from theline, said first range of angles being about 3 to 25 degrees, and saidsecond range being about 25 to 65 degrees, and said third range beingabout 65 to 85 degrees.
 14. The system of claim 9, said system furthercomprising an instrument causing relative rotational and translationalmotion between the beam and the surface, so that the beam scans a spiralpath on the surface.
 15. The optical system of claim 9, wherein thedevice provides a linearly polarized beam.
 16. An optical method fordetecting contaminants and defects of a test surface comprising:providing a polarized light beam along a path at an oblique angle to thetest surface, producing an illuminated spot thereon; collecting lightfrom the polarized light beam scattered by the surface and conveying thecollected light to a first detector; and collecting light from thepolarized light beam scattered by the surface and conveying thecollected light to a second detector, wherein the first and secondcollectors collect light scattered by the surface within differentranges of collection angles that are away from and that do not include aline normal to the test surface, said ranges of collection angles beingat different elevations from the test surface, wherein said firstcollector having an optical axis substantially along a lineperpendicular to the test surface.
 17. The method of claim 16, whereinthe collecting and conveying light to the first detector collects lightscattered from the spot within collection angles of about 3 to 25degrees from the line.
 18. The method of claim 16, wherein thecollecting and conveying light to the second detector collects lightscattered from the spot within collection angles of about 25 to 70degrees.
 19. The method of claim 16, said method further comprisingcausing relative rotational and translational motion between the beamand the surface, so that the beam scans a spiral path on the surface.20. The optical method of claim 16, wherein the providing provides alinearly polarized beam.
 21. The method of claim 16, wherein said methoddetects small anomalies in the presence of large anomalies on the testsurface.
 22. The method of claim 16, wherein said method enablesdifferentiation between large and small anomalies.
 23. The method ofclaim 16, wherein said method enables differentiation, characterizationand/or classification of anomalies, said anomalies comprising scratches,slip lines, crystal originated particles and particles by comparinglight collected by the two detectors.
 24. The method of claim 16,wherein said method enables differentiation between scratches, sliplines and crystal originated particles by comparing light collected bythe two detectors.
 25. The method of claim 16, wherein said methodenables differentiation between different topographic features bycomparing light collected by the two detectors.
 26. An optical methodfor detecting contaminants and defects of a test surface comprising:providing a polarized light beam along a path at an oblique angle to thetest surface, producing an illuminated spot thereon; collecting lightfrom the polarized light beam scattered by the surface within a firstrange of collection angles by a first collector, said first collectorconveying the collected light to a first detector; said first collectorproviding a single output in response to the light collected by it;collecting light from the polarized light beam scattered by the surfaceand conveying the collected light to a second detector, said secondcollector providing a single output in response to the light collectedby it; wherein the second collector collects light scattered by thesurface within a second range of collection angles that are at differentelevations from the first range of collection angles, at least one ofsaid first and second ranges of collection angles being away from anddo(es) not include any direction normal to the test surface, whereinsaid first collector having an optical axis substantially along a lineperpendicular to the test surface.
 27. The method of claim 26, whereinthe collecting and conveying light to the first detector collects lightscattered from the spot within collection angles of up to 25 degreesfrom the line.
 28. The method of claim 26, wherein the collecting andconveying light to the first detector collects light scattered from thespot within collection angles of about 3 to 25 degrees from the line.29. The method of claim 26, wherein the collecting and conveying lightto the second detector collects light scattered from the spot withincollection angles of about 25 to 70 degrees.
 30. The method of claim 26,said method further comprising causing relative rotational andtranslational motion between the beam and the surface, so that the beamscans a spiral path on the surface.
 31. The optical method of claim 26,wherein the providing provides a linearly polarized beam.
 32. The methodof claim 26, wherein said method detects small anomalies in the presenceof large anomalies on the test surface.
 33. The method of claim 26,wherein said method enables differentiation between large and smallanomalies.
 34. The method of claim 26, wherein said method enablesdifferentiation, characterization and/or classification of anomalies,said anomalies comprising scratches, slip lines, crystal originatedparticles and particles.
 35. The method of claim 26, wherein said methodenables differentiation between scratches, slip lines and crystaloriginated particles.
 36. The method of claim 26, wherein said methodenables differentiation between different topographic features.
 37. Anoptical system for detecting anomalies of a sample, comprising: opticsdirecting a beam of radiation along a path at an oblique angle onto asurface of the sample; at least one detector; and an optical devicereceiving scattered radiation from the sample surface and originatingfrom the beam and focusing the scattered radiation to said detector,causing said at least one detector to provide an output, said devicecomprising a curved mirrored surface collecting the radiation from thesample surface, said curved mirrored surface having an axis of symmetrysubstantially coaxial with a line normal to the surface so thatdirectional scattering information in the received scattered radiationrelative to the beam is retained.
 38. The system of claim 37, saidmirrored surface defining an input aperture positioned proximate to thesample surface to receive scattered radiation therethrough from thesample surface.
 39. The system of claim 38, said mirrored surface havinga paraboloidal portion, the paraboloidal portion reflecting radiationthat passes through the input aperture, said device further comprisingmeans for focusing radiation reflected by the mirrored surface to thedetector.
 40. The system of claim 38, said mirrored surface being anellipsoidal mirrored surface, the mirrored surface reflecting andfocusing radiation that passes through the input aperture.
 41. Anoptical method for detecting anomalies of a sample, comprising:directing a beam of radiation along a path at an oblique angle onto asurface of the sample; providing a curved mirrored surface to receivescattered radiation from the sample surface and originating from thebeam, said curved mirrored surface having an axis of symmetrysubstantially coaxial with a line normal to the surface so thatdirectional scattering information in the received scattered radiationrelative to the beam is retained; and detecting the scattered radiationfrom the mirrored surface by at least one detector to detect anomaliesof the sample.
 42. The method of claim 41, further comprising focusingradiation reflected by the mirrored surface to a detector.
 43. Anoptical system for detecting anomalies of a sample, comprising: opticsdirecting a beam of radiation along a path at an oblique angle onto asurface of the sample; at least one detector; and an optical devicereceiving scattered radiation from the sample surface and originatingfrom the beam and focusing the scattered radiation to said detector,causing said at least one detector to provide an output, said devicecomprising a radiation collector having an axis of symmetrysubstantially coaxial with a line normal to the surface so thatdirectional scattering information in the received scattered radiationrelative to the beam is retained.
 44. The system of claim 43, saidmirrored surface defining an input aperture positioned proximate to thesample surface to receive scattered radiation therethrough from thesample surface.
 45. The system of claim 44, said mirrored surface havinga paraboloidal portion, the paraboloidal portion reflecting radiationthat passes through the input aperture, said device comprising means forfocusing radiation reflected by the mirrored surface to the detector.46. The system of claim 43, said mirrored surface being an ellipsoidalmirrored surface, the mirrored surface reflecting and focusing radiationthat passes through the input aperture.
 47. An optical method fordetecting anomalies of a sample, comprising: directing a beam ofradiation along a path at an oblique angle onto a surface of the sample;providing a radiation collector to receive scattered radiation from thesample surface and originating from the beam, said collector having anaxis of symmetry substantially coaxial with a line normal to the surfaceso that directional scattering information in the received scatteredradiation relative to the beam is retained; and detecting the scatteredradiation from the collector by at least one detector to detectanomalies of the sample.
 48. The method of claim 47, further comprisingfocusing radiation reflected by the mirrored surface to a detector.